How old is a diamond? Do we put the zero time stamp at the point when the carbon atoms started to cluster together in the neat diamond arrangement, or when a violent volcanic eruption sends the stone speeding out of the earth and into the hands of humans? Or do we take the model of historians and auctioneers and say that the diamond formed at the moment when it was cut and mounted? Today’s news would suggest the latter, as a diamond reportedly “at least 400 years old” was sold by Sotheby’s for a tasty $9.7 million.

Geologically, the age of a diamond generally refers to the time when it was formed in the earth’s mantle (the bit between the crust and the core), which could be millions, hundreds of millions or even billions of years ago. Boffins with lasers and lab coats can analyse tiny inclusions within the diamonds and, using some form of mathematical wizardry (involving various types of radioactive-type dating), give us a time when the diamond formed. Other clever cookies can look at different inclusions and tell us when the diamond was sent up to the surface, by using the way in which gases and solids separate when the stone is removed from the deep-earth pressure cooker.

Diamonds are not easy to date. In order to analyse an inclusion within a crystal, it must be either cut up or drilled into, and the science is still in its infancy.

Generally, most brides, rappers and owners of 10 million dollar stones are unwilling to allow geologists to cut up their diamonds, so we may never know exactly when this diamond was formed. I am, however, willing to bet anyone up to one English pound that it was more than 400 years ago.